This invention relates generally to coffee brewing systems, and more particularly, to a brewing decanter having a unique broad based design for mating with a low temperature controlled warmer plate.
In the past, coffee makers normally used decanters formed entirely of glass or metal, or a combination of both. Both glass and metal have a high heat transfer coefficient with causes a substantial heat loss through the walls of the decanter. As a result of such high heat losses, it is necessary to maintain a relatively high temperature on the warmer element to maintain the coffee at a sufficiently high temperature for consumption.
Plastic decanters have also been employed in the past for use in coffee brewing machines. Since plastic has a lower heat transfer coefficient, it is a more desirable material for minimizing the heat loss through the side walls of the decanter. However, the rate of heat transfer through the bottom surface of the decanter from a relatively flat warmer plate is much lower than with a glass or metal decanter.
Since plastic decanters of the past exhibit a high resistance to heat transfer through the bottom wall, the warmer plate temperature necessary to maintain the coffee at the desired temperature potentially could melt or deform the plastic decanter. Further, heating of the coffee to such high temperatures in the localized area immediately adjacent the bottom wall has a detrimental effect on the taste of the coffee. Therefore, there exists a need for a coffee warmer plate and a plastic decanter having a low coefficient of heat transfer with a relatively large heating surface for cooperating with the warmer plate and for providing efficient heat transfer therebetween.
In addition, most decanters of the past do not include a cover or lid so that the pouring spout of the decanter is exposed to ambient air when the decanter is filled and stored with a quantity of warm coffee. As a result, a substantial amount of heat escapes from the coffee to the ambient air currents circulating across the opening of the pouring spout of the decanter. The heat losses are further increased when the coffee brewing machine is exposed to air currents in an air conditioned room or office building. The steam moisture escaping from the open pouring spout of the decanter carries with it the aroma and taste leaving the coffee stale and bitter in approximately thirty minutes.
It has been common practice in the past to manufacture coffee brewing machines with a plurality of warmierplates which are formed with upturned or knurled peripheral rims for retaining the coffee decanter generally centered thereon and for acting as a cooling vent for dissipating heat to the atmosphere. This design assists in maintaining the adjacent peripheral walls of the warmer plates at a relatively cool temperature. The surfaces adjacent to the warmer plates and the heating elements are identified as the "warming deck".
Generally, the upturned knurled peripheral rims of the warmer plates do not physically contact the warming deck, but form slots therebetween for limiting the surface temperature of the warming deck to less than 125.degree.-130.degree. F. Such a requirement is a safety related feature and must be satisfied before a seal of approval from an independent testing laboratory, for example, such as Underwriters Laboratories will be granted. However, such warmer plates are normally shallow and not designed to afford a close fit with the decanter to produce a sufficiently high rate of heat exchange therebetween for permitting the use of a plastic decanter.
Coffee brewing machines have been known in the past to include a carafe or decanter which was positioned on a metal warmer plate and which included an unturned ring portion within which the bottom of the carafe was closely received. A heating element was contained within a shroud mounted to the underside of the warmer plate. The carafe was comprised of a transparent plastic body equipped with a handle and pouring spout with the bottom of the carafe preferably a plate fashioned out of aluminum or other metal having high thermal conductivity. The plate was stake rolled and sealed in a leakproof fashion while the bottom of the carafe plate was formed in a manner for seating closely within the warmer plate ring. The flat underside of the carafe was seated flush on top of the flat surface of the warmer plate and was shaped to assure a close fit within the warmer plate ring for providing a maximum surface area contact with the warmer plate.
Decanters fabricated from the combination of the plastic upper portion and the thin walled metallic lower portion were also developed to help overcome the safety problems associated with glass decanters. A common problem is one in which an empty glass decanter is left on a warmer plate. What generally results is the rapid heating of the bottom glass surface which causes thermal expansion and loss of temper of the glass. The weakened bottom of the glass decanter can possibly break off after thermal expansion and loss of temper of the glass occurs. However, the weakened bottom of the glass decanter also could fracture and separate from the body of the decanter at some time later when the decanter is filled with coffee. While elminating some of the danger associated with glass decanters, the metal bottom of the plastic metal decanter actually accelerated the heat transfer causing rapid deterioration of the coffee flavor.
Heretofore, it was not generally recommended that plastic decanters be placed on warmer plates because of the relatively low heat transfer coefficient. It was believed that the plastic bottom would melt when placed on a warmer plate having a sufficiently high temperature to maintain the volume of coffee at the desired temperature. Typically, the warmer plate is a flush mounted flat unit which may be slightly concave such as a shallow dish for loosely receiving the bottom of the decanter. Because of this construction, air currents passing over the warmer plate will cause additional heat loss from the bottom of the decanter and from the surface of the warmer plate. Thus, the efficiency of the heat transfer from the warmer plate to the decanter is reduced.
Generally, coffee must be maintained at an optimum temperature in order to produce the proper flavor and taste. This optimum temperature is typically 180.degree.. Because of heat losses and inefficient heat transfer which occurs in existing coffee makers, the warmer g plate must often be kept at a temperature within the range of (350.degree.-400).degree. F. to maintain the coffee at the desired temperature. This results in hot spots being produced on the bottom of the decanter at the locations in which the decanter bottom contacts the heating element. This further results in the deterioration in the flavor of the coffee and becomes pervasive as the coffee circulates throughout the decanter by thermal convection caused by temperature gradients.
The hot spot problem is exacerbated when the warmer plate is formed of a material having a relatively low thermal conductivity. Because of the material employed in the construction of the warmer plate, heat generated by the heating element does not distribute evenly across the bottom of the decanter. Rather, the heat builds up in regions which are directly adjacent to the heating coils under the warmer plate. This problem also results in temperature gradients further deteriorating the flavor of the coffee.
Efforts in the past to solve this overheating problem have lead to the proposal that coffee warmers incorporate a water reservoir for maintaining the temperature of the water at about 200.degree. F. Since stored heat is dissipated by the coffee in the decanter by the convection transfer process, it was theorized that surrounding the decanter with 200.degree. F. water within the reservoir or jacket would maintain the coffee temperature at the desired level. However, use of a glass decanter with such coffee warmers resulted in dissipation of heat to the atmosphere at such a high rate that the temperature of the coffee fell below the palatable level.
Further, coffee decanters made of plastic material have been known for use in microwave coffee makers. For example, a coffee brewing machine for making coffee by the drip method in the past comprised a reservoir containing a thermally controlled valve adapted to open at a predetermined temperature. The thermally controlled valve was heated by water in a reservoir which was subjected to microwave radiation allowing the water to flow out of the reservoir and into and through coffee grounds in a filter beneath the valve. In that application, the selection of plastic materials for the decanter was dictated by a requirement for microwave transparency and not for thermal insulative characteristics.
Generally, decanters of the past including the decanter described in the copending patent application having Ser. No. 06/946,862 filed on Dec. 29, 1986 by Arthur L. Stoner entitled COFFEE MAKER WITH PLASTIC DECANTER AND LOW TEMPERATURE WARMING PLATE, have outwardly extending sides. The outwardly extending sides of the decanters of the past resulted in the volume of the liquid that was being heated was disposed well above the heating element. The extending sides of the decanters were exposed to cooling air resulting in loss of heat by convection heat transfer.
Commercial decanters of the past have been made of either glass or metal which have higher heat transfer coefficients resulting in substantial heat loss through the decanter walls. It was therefore necessary to maintain a relatively high temperature on the heating element to retain the coffee at approximately 180.degree. F. Since plastic decanters have a lower heat transfer coefficient than either glass or metal, the actual measurable heat transfer from the heating element to the coffee was lower for the plastic decanter. This required the heating elements to be maintained at approximately 380.degree. F. to provide coffee at approximately 180.degree.. However, plastic decanters of the past fashioned in conventional round configurations would soften and begin to melt at approximately 350.degree.. Therefore, plastic material was considered unsuitable for conventional round configurated decanters.
Hence, those concerned with the development and use of coffee brewing machines of the commercial variety have long recognized the need for improved plastic broad-based decanters which cooperates with a larger warming plate having a deeper concave construction for providing a closer fit with the plastic decanter permitting the use of lower heat levels and more efficient heating of the stored coffee and which reduces air convection heat losses and potential hazards to brewing machine operators. The present invention fulfills all of these needs.